Electro-optical sensors are known, comprising a plurality of photo-detector devices suitable to detect light signals by means of a photo-sensitive element, and to transmit them in the form of electrical signals to a calculator unit that processes them, obtaining images. The images are then transmitted to display devices which allow a user to see the images or information deriving from them.
Electro-optical sensors, generally based on silicon technology of the CMOS type (Complementary Metal Oxide Semiconductor), are able to offer a high quality image even in lighting conditions that vary greatly inside the same scene (see for example Seger, Graf, Landgraf—“Vision assistance in Scene with extreme contrast”—IEEE Micro, vol. 13 page 50, February 1993).
It is generally known that the quality of the images can be improved by compressing the signal on a logarithmic scale inside the photo-sensitive element. Solutions are known in which the compression is achieved by connecting to the photo-sensitive junction a MOS type transistor in a diode configuration, as described for example in U.S. Pat. No. 5,608,204.
One disadvantage of this type of electro-optical sensors is that, in the case of poor lighting, the definition of the image is not optimal.
A photo-sensitive element for electro-optical sensors is also known, from EP-A-1.770.985, developed to provide images with a high repetition frequency and high quality, both in cases of poor lighting and also when there is a high range of luminosity inside the same scene, that is, in so-called conditions of high light dynamics.
In the configuration proposed in EP-A-1.770,985, the photo-sensitive junction is polarized at a reference voltage value, called reset condition, set by the user, and then insulated at the start of the signal integration phase, acting on a digital type switch.
The photo-sensitive element proposed in EP-A-1.770,985 comprises a first linear output, of the conventional type, and a second output that allows to map information in the event of high luminosity. The functioning principle is based on the temporal mapping of the instant when the first output reaches the signal level beyond which it is no longer significant, called saturation level. To map this information correlated to the saturation level, a comparator is used which, at the instant of saturation, triggers a switch and freezes in a memory the current value of an external reference signal. This signal has a shape that can be programmed from the outside and is memorized on a capacitor. The signal is therefore used only as a temporal reference, exploiting the point-to-point correspondence between time and voltage.
One disadvantage of this photo-sensitive element is that it is complex, and requires a lot of space to be integrated on a silicon support or substrate, thus limiting the possibility of integration in a reduced space.
An alternative technique for obtaining a response with high light dynamics consists of returning the photo-sensitive junction to the reference voltage, called reset condition, during the integration time, every time the output of the photo-sensitive element reaches the saturation level, and of counting the event in a memory. One example of this kind of technique is found in WO2009/042901, where the total integration time is divided into intervals of different duration, programmable by the user. At the end of every interval, the output of the photo-sensitive element is controlled. If the output has reached the saturation level, the photo-sensitive junction is taken to the original reset status and the event is stored in a support memory; on the contrary, if the saturation level has not been reached, the integration is continued without modifying the status of the photo-sensitive element.
The main disadvantage of this technique is that it is necessary to associate with every photo-sensitive element a memory able to memorize the number of times that the photo-sensitive element has reached the saturation level, and a sophisticated digital control able to discriminate the need for a new reset of the photo-sensitive junction.
Among the various alternatives that exploit information contained inside the same scene there is also the known technique that uses a voltage generator for generating a plurality of reference voltages, in which the voltage generator is combined with a switch circuit for the selective opening/closing of the corresponding circuit. The selective activation of the switches allows to apply the voltage produced by the voltage generator to one and/or the other of plurality of pixels disposed in a matrix so as to selectively control the quantity of charge dissipated by the light falling on the photo-sensitive elements. The sequentiality of reference voltages generated is such as to expand the range of visible luminosity inside the same scene, irrespective of the value of capacities associated with the matrix of pixels.
Using this technique, which provides to use a single generator of reference voltages, has the disadvantage that it allows to polarize the pixels, or a subset of pixels of the matrix, exclusively at a voltage value common to the whole matrix and set by the reference voltage generator. Because of this, for example, it is not possible to manage the reset status of the individual pixel, or of a subset of selected pixels, independently of the status of the other pixels in the matrix. In fact, every single pixel can only be connected or disconnected to/from the single reference voltage by means of a system of switches, without any other possible configurations being provided. Consequently, the reset status of the pixels can only be managed overall with respect to the matrix or other arrangement that comprises them.
One purpose of the present invention is to obtain a photo-detector device for electro-optical sensors that can be integrated into a small-size silicon support element or substrate, achieving a microchip, and that is suitable to supply good quality images at high repetition frequency, both in cases of poor lighting and also when there is a wide range of visible luminosity inside the same scene.
Another purpose of the present invention is to obtain an electro-optical sensor that comprises a plurality of photo-detector devices in which pixels can be disposed according to a matrix, or in another desired disposition, and which allows to manage independently the reset status even of a single pixel, or a desired subset of pixels of the matrix, or other disposition, in order to improve the reading conditions thereof.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.